Prepare for dart 3.0 alpha changes: generate assets

[annagrin]

• Prepare or Dart 3 alpha breaking changes:
  • Generate missing SDK assets for tests.
  • Enable null safe tests that use the generated assets.

Towards: #1878
Helps: #1818
Closes: #1591

[nshahan]

LGTM pending the comments from @elliette.

[elliette]

I don't think the mono_pkg.yaml / mono repo changes should be part of this PR. It looks like we are missing mono_pkg.yaml files in some of our test fixtures, I think we should add those separately.

[elliette]

LGTM with a few remaining comments. Thanks!

[elliette]

I don't think the mono_pkg.yaml / mono repo changes should be part of this PR. It looks like we are missing mono_pkg.yaml files in some of our test fixtures, I think we should add those separately.

Ah, remembered those changes are being tracked here: #1845

[annagrin]

I don't think the mono_pkg.yaml / mono repo changes should be part of this PR. It looks like we are missing mono_pkg.yaml files in some of our test fixtures, I think we should add those separately.

Ah, remembered those changes are being tracked here: #1845

Since I had to move webdev tests to sound null safety, it does not seem reasonable to run the CI analyzer task on the unsound one. So I just switched the task to run on the sound project. (It also fixed an analyzer test breaking). It seems like part of this change logically. If we would like to analyze or test other test fixtures on CI we can add it separately.